1. Field of the Invention
This invention relates to a robot comprising a plurality of joint axes, links coupling the joints and actuators for driving the joint axes and adapted to operate by supplying a drive current to the actuators. More particularly, the present invention relates to a robot adapted to prevent an over-current from flowing to any of its actuators during an operation and also to an over-current protection device to be used for such a robot.
Still more particularly, the present invention relates to a robot adapted to prevent an over-current from flowing to any of its actuators when an excessively heavy load is applied to the related joint axes and also to an over-current protection device to be used for such a robot. Still more particularly, the present invention relates to a robot adapted to prevent an over-current from flowing to any of its actuators when a heavy load that exceeds the permissible limit of the entire robot is applied to any of its joint axes and also to an over-current protection device to be used for such a robot.
This application claims priority of Japanese Patent Application No. 2003-403932, filed on Dec. 3, 2003, the entirety of which is incorporated by reference herein.
2. Description of Related Art
A machine device adapted to move in a manner resembling to motions of human being by means of electric and/or magnetic operations is referred to as “robot”. It is said that the word “robot” derives from a Slavic word “ROBOTA (slave machine)”. In Japan, robots started to become popular in the later 1960s, although many of them were industrial robots such as manipulators and transfer robots installed for the purpose of automating productive operations of factories and saving man power. Research and development efforts have been paid in recent years on mobile robots that are equipped with movable legs. Expectations are high for such mobile robots in practical applications. Two-legged mobile robots modeled after human motions are referred to humanoid robots.
While two-legged robots that are based on erect bipedalism are accompanied by problems of instability and difficulty of attitude control and walk control if compared with crawler type robots and four-legged or six-legged robots, they provide advantages including that they can adapt themselves to walking surfaces with obstacles and undulations on the way such as unleveled ground, and discontinued walking surfaces such as staircases and ladders to be stepped up and down so that they can move in a flexible manner.
Legged mobile robots generally have a large number of degrees of freedom of joints and are adapted to realize motions of joints by means of motors of actuators. More specifically, the link of an arm or a leg, which is a structure member of a robot, is coupled at an end thereof to the output shaft of a motor by way of a reduction gear and at the other end thereof to another motor for driving the corresponding neighboring joint. Additionally, the rotary position and the quantity of revolution of each motor are taken out for servo control in order to reproduce a desired pattern of motion and also for attitude control.
Currently, servo motors are generally used for realizing certain degrees of freedom of each joint of a robot. This is because servo motors are easy to handle and compact and show a high torque besides they are highly responsive. Particularly, AC servo motors are brushless and maintenance-free motors and hence can be applied to joint actuators of various automotive machines that can move and work in an unmanned work space such legged robots adapted to freely walk around. An AC servo motor is formed by arranging a permanent magnet and a coil having a plurality of phases respectively at the rotor side and at the stator side so as to generate a rotary torque at the rotor by means of a sinusoidal flux distribution and a sinusoidal wave current.
A sophisticated legged mobile robot can typically autonomously walk and perform various operations by using the legs. Additionally, such a robot can stand up from a posture of lying on the face or on the back. It can also hold an object by hand and carry it to some other place. On the other hand, when such a robot falls on the floor or collides with an object on the floor on the intended route of movement, some or all of the joint actuators of the robot may be subjected to a tremendously excessive load.
The machine can be broken, plastically deformed or otherwise fatally damaged by such an excessive load. Therefore, it may be important for the motors of the joint actuators of a robot to incorporate a mechanism for absorbing such a load.
FIG. 1 of the accompanying drawings schematically shows a known robot, which is illustrated as a simple model. The robot drives a motor 120 under the control of a host controller (not shown) and the output torque of the motor 120 is applied to a link 122 by way of gear 121 so as to drive a movable part thereof.
In the illustrated instance, a torque limiter is arranged between the gear 121 and the link 122 so that it is possible to prevent the output shaft of the motor 120 from being deformed and the motor 120 from being otherwise damaged by any impact externally applied thereto as the torque limiter absorbs the impact.
Various torque limiters (or servo savers) that can be used for such applications have hitherto been proposed [see, inter alia, Japanese Utility Model Application Laid-Open Publication No. 60-192893 (FIGS. 1 and 2)]. FIG. 2 schematically illustrates a torque limiter of the type under consideration, showings its configurations.
The illustrated torque limiter 130 comprises semi-annular first and second friction plates 132A, 132B arranged in the inside of an annular body 131 that is rigidly fitted to a link 135. The first and second friction plates 132A, 132B are rigidly fitted to the output shaft 134 of a motor by way of resilient bodies 133, which may be pieces of rubber or compression coil springs. Then, the first and second friction plates 132A, 132B are pressed against the inner wall surface of the annular body 131 by the resilient bodies 133 under pressure of a predetermined level.
In the illustrated torque limiter 130, the annular body 131 can be rotated integrally with the output shaft 134 of the motor by the frictional force that is generated between the first and second friction plates 132A, 132B and the annular body 131 in normal operating conditions. On the other hand, when the annular body 131 is subjected to a large load as a result of the impact applied to the link 135 and the load is greater than the static frictional force between the first and second friction plates 132A, 132B and the annular body 131, the annular body 131 slides on the first and second friction plates 132A, 132B so that the output shaft 134 of the motor is not subjected to a load that is greater than the dynamic frictional force between the annular body 131 and the first and second friction plate 132A, 132B.
However, such conventional torque limiters 130 are accompanied by a problem that the coefficient of static friction between the annular body 131 and the first and second friction plates 132A, 132B shows a large dispersion among individual torque limiters if they are manufactured according to same numerical specifications so that it is difficult to establish a design margin for the entire robot in terms of the coefficient of static friction.
Additionally, with such conventional torque limiters 130, the coefficient of static friction between the annular body 131 and the first and second friction plates 132A, 132B can easily change as a function of temperature. Thus, it is difficult to handle them also from the viewpoint of the coefficient of static friction.
Still additionally, since conventional torque limiters 130 have a rather complex and bulky mechanical configuration as described, it is difficult to make them lightweight. Then, it is difficult to reduce the size and weight of a robot that comprises a large number of motors.
Furthermore, an over-current may flow to the actuators for driving some of the joints of a robot in an attempt to make the robot move according to the command given to it when an excessively large load torque is applied to any of the joints. Such over-current makes the actuators operate faultily.
For example, if individual actuators do not show any over-current by themselves, the total electric current flowing to a plurality of actuators can exceed the permissible level of the entire system. Particularly, in the case of a robot whose bodily components are driven to move in a coordinated manner under control, the entire system requires over-current protection because, when a joint is subjected to an overload, the excessive load is often distributed to all the body of the robot in order to avoid an over-current condition.